Relationship between heterogeneous changes in airway morphometry and lung resistance and elastance.

We present a dog lung model to predict the relation between inhomogeneous changes in airway morphometry and lung resistance (RL) and elastance (EL) for frequencies surrounding typical breathing rates. The RL and EL were sensitive in distinct ways to two forms of peripheral constriction. First, when there is a large and homogeneous constriction, the RL increases uniformly over the frequency range. The EL is rather unaffected below 1 Hz but then increases with frequencies up to 5 Hz. This increase is caused by central airway wall shunting. Second, the RL and EL are extremely sensitive to mild inhomogeneous constriction in which a few highly constricted or nearly closed airways occur randomly throughout the periphery. This results in extreme increases in the levels and frequency dependence of RL and EL but predominantly at typical breathing rates (<1 Hz). Conversely, the RL and EL are insensitive to highly inhomogeneous airway constriction that does not produce any nearly closed airways. Similarly, alterations in the RL and EL due to central airway wall shunting are not likely until the preponderance of the periphery constricts substantially. The RL and EL spectra are far more sensitive to these two forms of peripheral constriction than to constriction conditions known to occur in the central airways. On the basis of these simulations, we derived a set of qualitative criteria to infer airway constriction conditions from RL and EL spectra.